1. Field of the Invention
The invention relates to a photopolymerizable material that is suitable for the production of colored images, in particular, for color proofing for multicolor printing. The invention further relates to a process for the production of a colored image, in particular, a color proofing method in which a photopolymerizable material comprising a transparent film support, a colored polymerizable layer and a thermoplastic adhesion layer is laminated onto an image-receiving sheet, exposed imagewise through the film support and developed to produce the image by peeling off the film support together with the exposed areas of the colored polymerizable layer.
2. Description of Related Art
Materials and processes relating to color proofing are known and are described, for example, in U.S. Pat. Nos. 4,895,787 and 5,049,476, the disclosures of which are incorporated herein by reference in their entirety. The color proofing process described therein has the advantage that development of the image does not require the use of any alkaline or acidic solutions or organic solvents. Color proofing materials typically include a transparent film support, a colored photopolymerizable layer and a heat-activatable adhesion layer. A disadvantage of these materials is that they produce images whose background areas have a yellow discoloration. As has been found, this discoloration is predominantly attributable to the presence of the photoinitiator in the photopolymerizable layer which can diffuse into the adhesion layer during storage.
Because this photoinitiator also is present in the image areas of the photopolymerizable layer, the color of each individual color image is distorted by its presence. Although attempts have been made to employ photoinitiators whose absorption is exclusively in the ultra-violet region and which therefore appear colorless in the visible region, these photoinitiators are significantly less effective in the light of conventional copying lamps and require very long exposure times. It also has been found that even visually colorless photoinitiators can cause a color shift, since these compounds frequently form colored photodecomposition products.
U.S. Pat. No. 3,615,435 describes a process for the production of colored images in which a photosensitive material comprising a layer support, an uncolored photopolymerizable layer, a colored photopolymerizable layer and an image-receiving sheet is exposed image-wise and developed to produce an image by peeling apart the layer support and the image receiving sheet. The unexposed areas of the colored photopolymerizable layer remain on the image-receiving sheet, while the exposed areas remain on the layer support. Using this method, single-color images are obtained. The production of multicolored images from superimposed single-color images also is described, however, development is always carried out by washing out. A color shift also occurs in this material since the colored photopolymerizable layer contains a photoinitiator.
U.S. Pat. No. 4,288,525 describes a photosensitive material which contains an uncolored photopolymerizable layer and a colored, non-photosensitive layer. The material is exposed and developed by peeling apart. The photopolymerizable layer and at least part of the colored layer remain on the layer support in the exposed areas, while both layers remain on the image-receiving material in the unexposed areas. Color distortion therefore is caused in the colored positive image by the presence of the photoinitiator in the photopolymerizable layer. A similar material and process are described in U.S. Pat. No. 4,692,395. The disclosures of the U.S. Patents described above are incorporated herein by reference in their entirety.
Thus, there exists a need to provide a material and a photopolymerizable process that produces color images that are not distorted by photoinitiator residue and that have low background discoloration.